1. Field of the Invention
This invention relates to amplifier circuits and more particularly to differential transimpedance amplifier circuits.
2. Description of the Relevant Art
Fully balanced or differential transimpedance amplifiers are utilized in a variety of applications where it is desirable to convert a current-varying signal into a voltage-varying signal. One such application is within optical receiver systems where the transimpedance amplifier is used to convert the current-varying output signal of a photodetector into a voltage signal that is processed by other circuitry. Although fully balanced or differential transimpedance amplifiers are typically associated with favorable power supply rejection characteristics, they are also often characterized with a relatively low voltage gain, a large low-cutoff frequency, poor noise performance, limited dynamic range, and/or a low bandwidth.
A typical differential transimpedance amplifier circuit for an optical receiver system is shown in FIG. 1. The amplifier includes a differential pair of transistors 10 and 11 and source follower transistors 13 and 14 connected through level shifting diodes 16 and 17. Feedback resistors 19 and 20 couple output voltages V.sub.01 and V.sub.02 to the gate inputs of transistors 10 and 11 to broadband the amplifier (i.e. increase the bandwidth) and stabilize the DC bias point. One terminal of a photodetector 22 (typically a metal-semiconductor-metal (MSM) detector) is DC coupled to the gate of transistor 10 while a second terminal is AC coupled to the gate of transistor 11 with a blocking capacitor 23. This coupling allows the photocurrent from photodetector 22 to circulate partially through resistors 19 and 20 to produce a balanced differential output voltage- Noise on power supply V.sub.DD is partially common-mode and is partly rejected by the next differential stage- Bias resistor 25 establishes the DC voltage at the second terminal of photodetector 22 to provide the required bias voltage.
The amplifier circuit of FIG. 1 is differential and thus provides a moderate amount of power supply rejection. However, the amplifier has a relatively low voltage gain, a relatively large low-cutoff frequency, poor noise performance, limited dynamic range, and/or low bandwidth. The voltage gain is low because the voltage drop across the load resistors 27 and 28 is small- The low-cutoff frequency is high because the on-chip capacitor 23 is small. The noise performance is poor because the voltage gain of the input stage is low (more proportional noise contribution from resistors 19, 20, 27 and 28, and the second stage) and because bias resistor 25 is usually small. To achieve high bandwidth, resistors 19 and 20 must be small because of the low voltage gain. This degrades noise performance as mentioned previously. If resistors 19 and 20 are chosen to be large for good noise performance, the bandwidth will be low because of the low voltage gain.
A differential transimpedance amplifier that rejects power supply noise, that has a relatively large voltage gain (to improve both the noise performance and bandwidth), that has a small low-frequency cutoff, and that has a means for establishing the DC bias potential across the input photodetector without the use of a bias resistor (which degrades noise performance and limits dynamic range) is therefore desirable.